The present invention relates to a data transmission system using an automatic repeat request (hereinafter is referred to as abbreviation "ARQ") and more particularly to a data transmission system for realizing a procedure of retransmission of error detected frames, which is adapted to a transmission channel such as radio communication with relatively high transmission error-rate.
The recovery of transmission error in data transmission has been conducted by automatic retransmission using ARQ and/or by error-correction technique using error-correction code. ARQ is a method of recovering an error in a data transmission in which any blocks of data found to contain errors at the receiving terminal are automatically requested for repeat transmission from the transmitting terminal. In response to this request, the required blocks are transmitted again from the transmitting terminal to the receiving terminal. A Go-back-N ARQ method and a selective repeat ARQ method are representative.
An example of procedure of the Go-back-N ARQ method is as follows:
In this method, the transmitting terminal transmits data with additions of an error detection code and transmission sequential number, and the receiving terminal performs error checking on every frame of the received data. When any frame containing an error is detected, the recieving terminal informs the transmitting terminal of the sequential number of the frame concerned and requests repeat transmission of the data from the transmitting terminal. The transmitting terminal searches back the sequential number of the frame and repeats transmission of the sequence of data starting from the required frame.
The detailed procedure of retransmission of frames by the Go-back-N ARQ method is as follows:
The transmitting terminal transmits data with error detection codes and transmission sequence numbers N(S). At the receiving terminal, a frame with transmission sequence number N(S)=2 is now found to contain an error. In this case, the receiving terminal transmits a REJ frame with a numeral 2 previously set in a received sequence number N(R) for requesting repeat transmission of the frame having the sequence number 2. Upon receipt of the REJ frame, the transmitting terminal returns to transmission number N(S)=2 and repeats transmission of data in the sequence beginning from the specified sequence number. However, this method has relatively low efficiency and reduces a throughput in data transmission since it requires repeat transmission of the already transmitted correct frames following the requested frame.
The selective repeat ARQ method realizes a higher efficient retransmission. The procedure of this method is as follows:
According to this method, a transmitting terminal transmits data provided with codes for error-detection and transmission sequence numbers. A receiving terminal performs error checking on every frame of the received data any frame containing an error is detected, it informs the transmitting terminal of the sequential number of the frame concerned and requests repeat transmission of the frame from the transmitting terminal. The transmitting terminal repeats transmission of the requested frame only.
The detailed procedure of retransmission of frames is achieved by using the selective repeat ARQ method. The transmitting terminal transmits data with error detecting codes and transmission sequence numbers N(S). At the receiving terminal, a frame with transmission sequence number N(S)=2 is now found to contain an error. In this instance, the receiving terminal transmits a SR frame with a numeral 2 previously set in a received sequence number N(R) for requesting retransmission of the frame having the sequence number 2. Upon receipt of the SR frame, the transmitting terminal transmits the required frame having the sequence number N(S)=2 only and then starts transmission of data in the sequence from the sequence number next to a sequence number already transmitted normally (N(S)=6).
However, the frame (REJ frame or SR frame) for requesting repeat transmission of an error frame may also be incorrectly transmitted over a data transmission path in which transmission error may frequently arise. When the transmitting terminal could not correctly receive the REJ frame (of the Go-back-N AQR method) or the SR frame (of the selective repeat ARQ method) due to the transmission error, the receiving terminal must transmit again the REJ frame or SR frame after a specified time elapsed. This results in further decreasing the data transmission efficiency and reducing the throughput in the data transmission.